Federal motor vehicle standards require that tail, brake and turn signal lamps be mounted on the rear of all trailers. Since trailers generally have no power sources, it is necessary to tap into the towing vehicle's power system to provide such power, particularly in correspondence with the energization of the lamps on the towing vehicle. Thus when the towing vehicle driver applies pressure to the vehicle's foot brake, not only are the vehicle's brake lights illuminated, but it is required that the trailer's brake lights also shall be illuminated, and be extinguished when the vehicle's brake lights are extinguished. The same applies to the turn signal lights and tail lights.
In some instances, where a vehicle light system includes a master separable connector, a T-tap connector device such as shown in U.S. Pat. No. 4,842,524 may be used. However, in many cases no such separable connector is present. In such cases, it has been necessary to locate each individual wire (such as for tail light, brake light and turn signal light) by a trial and error method, generally requiring two persons, one to activate the appropriate light from the driver's position and the other to test the wiring at the rear of the vehicle. After the appropriate wire is determined it may be tapped by any of a number of ways. One way is to cut the wire, strip its ends of insulation, and rejoin the ends with a third wire leading to the trailer. Another way is to remove insulation from a short section of wire, and wrap and solder the third wire to the bared wire. Still another way is to use an insulation-displacing tap, as shown in U.S. Pat. No. 4,941,844 and sold by 3M Company.
All these methods have the drawback of requiring that the wire related to each function (tail light, brake light, turn light) be identified, requiring two persons, and then providing a third wire and connecting it to the corresponding wire of the trailer wiring harness, after similarly identifying it. Such connections are time-consuming, require considerable skill and are not always reliable.